1. Field of the Invention
The present invention is directed to a hydraulic control device for controlling a boom-arm combined operation in an excavator and more specifically to a hydraulic control device adapted for control of a boom-arm combined operation that provides improved boom operation performance when an excavator performs works such as a loading-on-truck, a ground leveling and the like through the combined operation of a boom and an arm.
2. Description of the Related Art
Hydraulic excavators are provided with front work devices including a boom, an arm and a bucket, which can be actuated independently or in combination by a hydraulic flow discharged from one or more hydraulic pump to conduct a variety of works such as a digging, a ground leveling, a loading-on-truck and the like. As used herein, the term “combined operation” refers to an operation that is performed by simultaneous actuation of two or more of a boom cylinder, an arm cylinder and a swing motor.
As is known in the art, the conventional hydraulic excavators include a hydraulic control device, one example of which is illustrated in FIG. 1. The hydraulic control device is provided with a control valve having boom high-speed and low-speed control spools 101a, 101b and arm high-speed and low-speed control spools 103a, 103b that can be shifted to cause a boom and an arm to move at a relatively low speed (“low speed”) or relatively high speed (“high speed”) depending on the kind of operations to be carried out. The boom high-speed control spool 101b and the arm low-speed control spool 103a, which belong to a first spool group, are in fluid communication with a first pump P1 (hereafter, referred as “first pump”), while the boom high-speed control spool 101a and the arm low-speed control spool 103b, which belong to a second spool group, are in fluid communication with a second hydraulic pump P2 (hereafter, referred as “second pump”). The hydraulic flow generated by only one of the hydraulic pumps P1, P2 is used in actuating an arm cylinder 107 or a boom cylinder 109 at the low speed. On the contrary, the hydraulic flow generated by the first pump P1 and the hydraulic flow discharged by the second pump P2 are joined together in order to actuate the arm cylinder 107 or the boom cylinder 109 at the high speed. The flowing directions of the hydraulic flow are controlled by means of the respective control spools 101a, 101b, 103a and 103b. 
In the meantime, the boom is equipped at its distal end with a reinforcing structure and various kinds of attachments with a heavy coupling device, such as a bucket or the like. This increases the weight of the boom, as a result of which the load pressure exerting on the boom cylinder 109 soars up. Thus, in case of conducting the works through a combined operation of the boom and the arm, the hydraulic flow of the pumps P1, P2 is supplied for the most part to the arm cylinder 107 whose load pressure is lower than that of the boom cylinder 109, thereby disturbing the operation of the boom and hence making it difficult to perform the combined operation in a desired manner.
Particularly, in the event that the boom and the arm are fully extended to carry out a ground leveling work, the load pressure of the boom cylinder 109 is increased to a great extent and the hydraulic flow of the pumps P1, P2 is first fed to the arm cylinder 107 to which a relatively low load pressure is exerted, which means that the boom cylinder 109 is not supplied with a sufficient amount of the hydraulic flow. This may cause the arm to abruptly descend and strike the ground, thus hindering the excavator from smoothly performing the ground leveling.
In an effort to overcome such drawbacks, the hydraulic control device shown in FIG. 1 further includes a boom priority valve 110 disposed on a parallel line 105 that interconnects the second pump P2 and the arm low-speed control spool 103b lying downstream of the boom high-speed control spool 101a. The boom priority valve 110 serves to, in case of boom-arm combined operation, throttle or disconnect the parallel line 105 so that the hydraulic flow of the second pump P2 can be supplied to the boom high-speed control spool 101a in the first place to thereby move the boom faster than the arm.
As can be seen in FIG. 1, the boom priority valve 110 is a solenoid-operated valve having an invariable orifice 111 of fixed-opening area and position-controlled by turning on or off a switch 112. With this boom priority valve 110, if an operator wishes to have the boom move faster or slower than the arm, the switch 112 must be manually activated in correspondence to a desired operation mode, which makes the operator feel cumbersome.
Furthermore, when shifted to a position of the invariable orifice 111, the boom priority valve 110 throttles the parallel line 105 with a constant opening area, in which condition the operating speed of the arm fluctuates in response to variation of the load pressure imparted to the boom cylinder 109. This poses a problem in that the bucket carried at a distal end of the arm does not move in a horizontal direction but hits vertically onto the ground particularly at beginning of the ground leveling work.
Another example of the hydraulic control device for controlling a boom-arm combined operation in an excavator is disclosed in Japanese Laid-open Patent Publication No. 2000-96629. This hydraulic control device is designed to supplementally supply the hydraulic flow of a second pump with the hydraulic flow of a first hydraulic pump in proportion to the differential between an arm control pilot pressure and a boom control pilot pressure, in case that an excavator performs the combined work through a boom-arm combined operation. According to the hydraulic control device disclosed in the '629 publication, there may occur such an instance that the boom-arm combined operation is not conducted smoothly, because the operating speed of a boom varies with the working load applied to an arm.